Mankind has suffered heavily from earthquakes. Even in last several years, the Great Hanshin-Awaji Earthquake (Japan), Izmit Earthquake (Turk), Chichi Earthquake (Taiwan), and the like damaged utility life lines. Especially, in the Great Hanshin-Awaji Earthquake, three days were required to recover power and several months for city gas. This made people realize that comprehensive examination was necessary for the safety of utilities. Recently, taking advantage of lessons given by, e.g., the destruction of highways in the Great Hanshin-Awaji Earthquake, various earthquake-proof reinforcement methods have been examined to improve the strengths of constructions in case of an earthquake, including existing buildings and structures built under the old earthquake-proof design standard, assuming a greater earthquake.
For example, as earthquake-proof reinforcement methods for existing steel framed reinforced concrete structures, a method of winding a reinforcing bar and additionally placing concrete to improve the strength of a column member, a design method of winding a steel plate to reinforce a column member, and the like have been proposed.
However, in conventional design techniques applied to a pipe rack and the like, members such as beams, columns, or braces are selected without taking an ultimate load (true strength) until the pipe rack finally breaks into consideration. That is, for the relationship between a structure and an external force that the structure should withstand, a member with or without a sufficient strength may be selected, and no quantitative evaluation is reflected at the stage of design.
As a problem in the conventional earthquake-proof reinforcement method for a rack structure, when a steel column base and foundation are rigidly connected by jointing a reinforcing fitment to the joint portion between the column and a beam at the lower end of the steel column or applying reinforced concrete with pedestal foundation, the column base support method by the foundation changes from pin support to a fixed condition, and a shearing force or bending moment is transmitted to the foundation through the new interface between the column base and the reinforcing fitment or reinforced concrete. The shearing force or bending moment transmitted to the foundation increases with increasing magnitude of a seismic force. Hence, if the steel column base and the foundation are rigidly connected, the foundation itself cannot withstand a force more than allowable stress intensity and may break.
Conventional strength design for a foundation ensures a sufficient safety factor against an axial force (a load that acts in the axial direction of a member), though the safety factor against a bending moment has no sufficient margin as compared to that for an axial force. Hence, when the allowable stress of a foundation has some margin, the earthquake-proof reinforcement method of applying a cover plate to a column or beam can be preferably used. However, when the allowable stress of a foundation is designed with little margin, the method of simply reinforcing a column or beam using a cover plate is not preferable.
In the conventional pipe rack design, members such as beams, columns, or braces are selected without taking an ultimate load (true strength) until the pipe rack finally breaks into consideration. That is, for the relationship between a rack structure and a load that the structure should withstand, members having a sufficient strength and members without any sufficient strength may be consequently selected and mixed, and quantitative evaluation for load condition and member selection in consideration of situations until the entire pipe rack collapses is not reflected at the stage of design.
In addition, it is very important for structure design to quantitatively estimate the ultimate load of a pipe rack whose earthquake-proof performance is improved by extending the elastic region and plastic deformation region by earthquake-proof reinforcement. However, it is difficult to quantitatively evaluate this factor, and this evaluation has been done based on designer's experience.